cache: add tier definitions and EngineTier function

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/cache/tiers.go

@@ -0,0 +1,56 @@
+package cache
+
+import "time"
+
+// TTLTier represents a cache TTL tier with a name and duration.
+type TTLTier struct {
+	Name     string
+	Duration time.Duration
+}
+
+// defaultTiers maps engine names to their default TTL tiers.
+var defaultTiers = map[string]TTLTier{
+	// Static knowledge engines — rarely change
+	"wikipedia":     {Name: "static", Duration: 24 * time.Hour},
+	"wikidata":      {Name: "static", Duration: 24 * time.Hour},
+	"arxiv":         {Name: "static", Duration: 24 * time.Hour},
+	"crossref":      {Name: "static", Duration: 24 * time.Hour},
+	"stackoverflow": {Name: "static", Duration: 24 * time.Hour},
+	"github":        {Name: "static", Duration: 24 * time.Hour},
+
+	// API-based general search — fresher data
+	"braveapi": {Name: "api_general", Duration: 1 * time.Hour},
+	"youtube":  {Name: "api_general", Duration: 1 * time.Hour},
+
+	// Scraped general search — moderately stable
+	"google":     {Name: "scraped_general", Duration: 2 * time.Hour},
+	"bing":       {Name: "scraped_general", Duration: 2 * time.Hour},
+	"duckduckgo": {Name: "scraped_general", Duration: 2 * time.Hour},
+	"qwant":      {Name: "scraped_general", Duration: 2 * time.Hour},
+	"brave":      {Name: "scraped_general", Duration: 2 * time.Hour},
+
+	// News/social — changes frequently
+	"reddit": {Name: "news_social", Duration: 30 * time.Minute},
+
+	// Image search
+	"bing_images":  {Name: "images", Duration: 1 * time.Hour},
+	"ddg_images":   {Name: "images", Duration: 1 * time.Hour},
+	"qwant_images": {Name: "images", Duration: 1 * time.Hour},
+}
+
+// EngineTier returns the TTL tier for an engine, applying overrides if provided.
+// If the engine has no defined tier, returns a default of 1 hour.
+func EngineTier(engineName string, overrides map[string]time.Duration) TTLTier {
+	// Check override first — override tier name is just the engine name
+	if override, ok := overrides[engineName]; ok && override > 0 {
+		return TTLTier{Name: engineName, Duration: override}
+	}
+
+	// Fall back to default tier
+	if tier, ok := defaultTiers[engineName]; ok {
+		return tier
+	}
+
+	// Unknown engines get a sensible default
+	return TTLTier{Name: "unknown", Duration: 1 * time.Hour}
+}